Optoelectronics Materials and Techniques Part 7 pptx

Scanning electron microscope images of as-prepared titanium dioxide films on indium-doped tin oxide and fluorine-doped tin oxide FTO films coated glass substrates are shown in Fig.. Afte

contains an interfacial layer on the silicon surface However, in the present work, we could

not observe any interfacial layer on the silicon surface (Fig.5) Figure 5 shows the

high-resolution transmission electron microscopy (HRTEM) image of a ZnSe/Si heterostructures,

which reveals a clear interface between substrate (silicon) and overlayer (zinc selenide

layer) The main reason is the existence of a laterally varying potential barrier height, caused

-24 -22 -20 -18 -16 -14 -12

Fig 4 Forward and reverse current versus voltage characteristics of ZnSe/Au Schottky

diode The inset of Fig.4 shows the plot of voltage versus LnI [Reprinted with permission

from (Venkatachalam et al., 2006) Copyright @ IOP Publishing Ltd (2006)]

The reverse bias characteristics would be controlled by the generation-recombination and

band–to- band tunneling mechanisms at small (up to -0.4 V) and large bias, respectively,

which might be the reason for a small kink at –0.4 V (Chiang & Bedair, 1985) The plot

between the measured values of capacitance and voltage for ZnSe/ p-Si diodes is shown in Fig 6a

We obtained a straight line by plotting a curve between 1/C2 versus V, which implies a

similar behaviour for an abrupt heterojunction (Khlyap & Andrukhiv, 1999) The intercept

of this plot at 1/C2 = 0 corresponds to the built-in potential Vbi, and is found to be 1.51 V

The value of barrier height (Singh et al., 1993; Pfister et al., 1977) can be calculated from the

where V n = kT/q Ln (Nv/NA), k is the Boltzmann constant, T is the temperature, q is the

charge of the electron, Nv is the density of states in the valence band and NA is the effective

carrier concentration From the slope of the 1/C 2 versus voltage plot, the value of effective

carrier concentration is calculated as 3.55 × 1019 (m2/F)2 / V The calculated values of barrier

height and acceptor concentration (NA) are calculated as 1.95 eV and 4.37 × 1011 cm-3,

respectively The spectral photoresponse of the device prepared at 589 K is shown in Fig 6b

It shows a very good photoresponse in the UV-Visible range The quantum efficiency for

the device prepared at 553 and 589 K is calculated as 0.25 and 0.1 %, respectively

Fig 5 High-resolution transmission electron microscopy image of the prepared ZnSe/p-Si Schottky diodes [Reprinted with permission from (Venkatachalam et al., 2006) Copyright @ IOP Publishing Ltd (2006)]

3.2 Preparation and characterization of indium-doped tin oxide thin films

Nanocrystalline indium-doped tin oxide (ITO) thin films were prepared on glass and clay substrates by ion beam sputter deposition method Preparation and deposition parameters

of nanocrystalline indium-doped tin oxide thin films were found elsewhere (Venkatachalam et al., 2010) The scanning electron microscope (SEM) images show that the surface morphology of indium-doped tin oxide thin film on glass substrate is smooth (Fig 7a); in contrast, the surface morphology of indium-doped tin oxide thin film on clay substrate is rough (Fig 7b) The inset of Figure 7b shows the flexibility of indium-doped tin oxide thin film coated clay substrate Flexibility of indium-doped tin oxide thin film coated clay substrate is estimated as 17 mm, from a diameter of curvature X-ray diffraction patterns of annealed indium-doped tin oxide thin film are

shown in Fig 8; the X-ray diffraction patterns showed two different orientations, i.e., (400) and (222)

on different substrates, i.e., glass and clay, respectively The sheet resistances of indium-doped tin oxide thin film on glass (32 Ω/…) is lower than that on clay (41 Ω/…); it is due to the difference in substrate surface roughness between ITO/glass and ITO/clay

Fig 7 Scanning electron microscope images of indium tin oxide thin films (inset Fig 7b shows photograph of flexible ITO/Clay substrate) [Reprinted with permission from

(Venkatachalam et al., 2011) Copyright @ The Japan Society of Applied Physics (2011)]

3.3 Preparation and characterization of nanostructured titanium dioxide films

The hydrothermal synthesis of titanium dioxide (TiO2) film was carried out in a Teflon-lined stainless steel autoclave In a typical synthesis process, titanium n-butoxide (1.0 ml) was used with hydrochloric acid (20 ml) and deionized water (40 ml) The reaction time and temperature were fixed at 17 h and 160°C, respectively Scanning electron microscope images of as-prepared titanium dioxide films on indium-doped tin oxide and fluorine-doped tin oxide (FTO) films coated glass substrates are shown in Fig 9 It shows that the surface morphology of titanium dioxide films on indium-doped tin oxide substrate indicates

the existence of many uniform, dandelion-like structures with diameter in the range of 6-7

μm (Fig 9a) A selected area of high magnification image (inset of Fig.9a) shows that each dandelion-like structure is composed of nanorods with an average diameter of 150 nm It is attributed that if there is no lattice match between titanium dioxide and indium-doped tin oxide substrate, the titanium dioxide initially nucleates as islands and then the nanorods grow from these islands to form dandelion-like morphology In contrast, the surface morphology of titanium dioxide films on fluorine-doped tin oxide substrate (Fig 9c) shows that the whole surface is composed of ordered titanium dioxide nanorods with square top facets The cross-sectional view (inset of Fig.9c) confirms that the growth of the titanium dioxide nanorods is along the direction perpendicular to the fluorine-doped tin oxide substrate This shows that titanium dioxide thin film grows epitaxially on fluorine-doped tin oxide substrate; it is due to the small lattice mismatch (∼ 2 %) between titanium dioxide and fluorine-doped tin oxide films, because fluorine-doped tin oxide films and titanium dioxide films have similar crystal structure The length and size of the nanorods are evaluated as 3.9

indium-(211) peaks indicate that the nanostructured titanium dioxide film is highly oriented with respect to the substrate surface and the titanium dioxide nanorods grow in the (002) direction with the growth axis parallel to the substrate surface normal (Bang & Kamat, 2010)

After preparing the freestanding nanostructured titanium dioxide films, it is transferred from a glass substrate onto an indium-doped tin oxide film coated transparent flexible clay substrate The photograph of freestanding layer of titanium dioxide prepared by hydrothermal method is shown in Fig 10a; it can be easily handled with tweezers Figure 10b shows the scanning electron microscope images of freestanding titanium dioxide layer The size of the nanorod is calculated as 150 nm A very thin layer of titanium dioxide paste is used between the freestanding titanium dioxide and indium-doped tin oxide film coated flexible clay (LiSA-TPP) substrate in order to improve the adhesion The freestanding titanium dioxide layer deposited on flexible ITO/clay substrate is used as an anode The platinum sputtered indium-doped tin oxide film coated flexible clay/mica substrate is used

as a counter electrode Surlyn spacer film with a thickness of 60 μm is used as a spacer The completed device had an active area of 0.5 cm2 From the photocurrent density-voltage characteristic, the open circuit voltage, short circuit current and fill factor are calculated as 0.51 V, 1.14 mA and 56 %, respectively However, the efficiency of the prepared device is less than 1 % It is considered that the adhesion layer restricts the flow of electrons from titanium dioxide photoelectrode into the collector (ITO) (Park et al., 2011)

Fig 10 SEM images and photograph of freestanding TiO2 layer

3.4 Preparation of titanium dioxide nanotube arrays and titanium dioxide nanowire covered titanium dioxide nanotube arrays on titanium foil and plate

Nanostructured titanium dioxide films were prepared by anodization of titanium foil and plate at room temperature The anodization was performed in ethylene glycol containing

2 vol.% H2O+ 0.3 wt.% ammonium fluoride (NH4F) for different anodization time The anodized titanium sample was then annealed in air at 400°C for an hour Figure 11(a-d) shows top and bottom-side view scanning electron microscope images of anodized titanium plate and foil It clearly shows the formation of well-ordered titanium dioxide nanotube arrays on both titanium plate and foil The bottom side-views of the tube layer (Figs 11c and d) reflects an uneven morphology At the bottom, the tubes are closely packed together The diameter and length of titanium dioxide nanotube arrays on Ti plate are calculated as 100 nm and 5.6 μm, respectively

Figure 11(e-h) shows the X-ray diffraction patterns of anodized titanium plate and Ti foil before and after annealing In Fig 11e and f, the X-ray diffraction peaks at 35.3, 38.64, 40.4, 53.2 and 63.18 correspond to titanium This is attributed that the as-prepared titanium dioxide is amorphous before annealing; only titanium peaks are seen (Fig.11e and f) In order to change the amorphous titanium dioxide into anatase titanium dioxide, anodized titanium sample was annealed in air at 400°C for an hour After annealing, the amorphous titanium dioxide has been changed into crystalline with a more preferred orientation along (101) direction The particle size values of titanium dioxide on titanium plate and titanium foil are calculated as 41 and 24 nm, respectively The calculated lattice parameters of TiO2/Ti plate and TiO2/foil coincide well with the reported value of bulk titanium dioxide (a=3.7822Å) (JCPDS #21-1272) The stress in the TiO2/Ti plate is tensile On the other hand, the TiO2/Ti foil is under compressive stress (see Table 1)

Lattice parameter (a) (Å)

Stress (%)

Table 1 Structural parameters of anodized Ti plate and foil

Figure 12A and D shows the scanning electron microscope images of titanium dioxide nanowires covered titanium dioxide nanotube arrays prepared by anodization method The nanotubes divided into several parts are observed near the mouth (Fig.12C) The electrochemical etching causes the divided nanotubes to further split into several parts that lead to the formation of nanowires Figure 12B shows that titanium dioxide nanotube arrays with diameter of 100 nm exist underneath the nanowires

Figure 13 shows the photocurrent density-voltage characteristics of dye-sensitized solar cells based on titanium dioxide nanotube arrays and nanoparticles Under backside illumination, the short-circuit current density and power conversion efficiency of dye-sensitized solar cells based on titanium dioxide nanotube arrays are much higher than that of P25 (see Table

2) Similar results have been observed by (Tao et al., 2010) This result shows that the main factor responsible for the enhancement of the short circuit current is the improvement of electron transport and electron lifetime in titanium dioxide nanotube arrays This increased light-harvesting efficiency in titanium dioxide nanotube-based dye-sensitized solar cell could be a result of stronger light scattering effects that leads to significantly higher charge collection efficiencies of nanotube-based dye-sensitized solar cells relative to those of nanoparticles-based dye-sensitized solar cells (Jennings et al., 2008) The dye-sensitized solar cells device performance under backside illumination is very low This is attributed that the backside illumination affects the light absorption capacity of the dyes, because the I3- electrolyte cuts the incident light in the wavelength range of 400 – 650 nm

Fig 12 Scanning electron microscope images of anodized Ti foil and Ti plate Top views of Ti foil (A) and plate at low (C) and high magnification (D)]; cross-sectional view of Ti foil (B)

-4.0x10-3

-2.0x10-3

0.02.0x10-3

Fig 13 Photocurrent density-voltage characteristics of dye-sensitized solar cells based on TiO2 nanotube arrays and nanoparticles

Sample code Anodization Time (min) V (V) oc (mA/cm J sc 2 ) FF Efficiency (%)

Sample 1 (Ti Plate) 240 0.470 4.85 0.463 1.06

Sample 2 (Ti Foil)

Table 2 Photovoltaic parameters of dye-sensitized solar cells based on titanium dioxide

nanotube arrays and P25 films

3.5 Preparation of titanium dioxide nanotube arrays on indium-doped tin oxide and

silicon substrates

From the previous results, we observed that the use of foil and plate limits their potential applications, particularly in the fabrication of solar cells An alternative approach is the preparation of nanostructured titanium dioxide films on transparent conducting glass substrate by anodization method In the electrochemical anodization process, the substrate temperature, lattice mismatch between the substrate and film, and film thickness affect the properties of the films; because of which the anodization process is affected (Sadek et al., 2009) (Wang & Lin, 2009) reported that the formation of titanium dioxide nanotube arrays were not only affected by electrolytes and applied potential, but also affected by electrolyte

temperature Recently, titanium dioxide nanotube array films were successfully prepared by

anodization of as-prepared ion-beam sputtered titanium thin films at low electrolyte temperature (5°C) and the key parameter to achieve the titanium dioxide nanotube arrays is the electrolyte temperature (Macak et al., 2006) In the present work, the titanium dioxide nanotube arrays are successfully prepared by anodization of as-prepared ion-beam sputtered titanium films at room temperature Titanium thin films were deposited on indium-doped tin oxide and silicon substrates by ion beam sputter deposition method at room temperature The acceleration voltage supplied to main gun was fixed at 2500 V Pure

Ar was employed as the sputtering gas Nanostructured titanium dioxide thin films were prepared by electrochemical anodization method The Ti/ITO/glass was anodized in glycerol containing 2.5 vol % H2O+0.5 wt.% NH4F at an applied potential of 30 V for the anodization time of 240 min On the other hand Ti/Si sample was anodized in ethylene glycol containing 2.0 vol % H2O + 0.3wt % NH4F at an applied potential of 20 V for 180 min Nanostructured titanium dioxide thin films are formed by anodization using a two electrode configuration with Ti film as an anode and platinum as a cathode

Generally, the formation mechanism of the titanium dioxide nanotube array films is proposed as two competitive processes, electrochemical oxidation and chemical dissolution From these results, we observed that no titanium dioxide nanotubes, but titanium dioxide nanoholes were formed for anodization time of 60 min (Figure not shown) It shows that the titanium dioxide nanohole array films are easily formed during the short-time of anodization Titanium dioxide nanotube arrays can also be prepared on the titanium film surface, but this can be accomplished by increasing the anodization time; this is due to the

high chemical dissolution at the inter-pore region These results clearly show that high dissolution rate at the inter-pore region is very important in order to get the ordered nanotube arrays Figure 14 shows the top-view scanning electron microscope images of titanium film anodized in different electrolytes at 30 and 20 V for anodization time of 240 and 180 min, respectively It can be found that the pore growth and formation of titanium dioxide nanotube arrays on the titanium film surface are uniformly distributed Scanning electron microscope images confirm the formation of titanium dioxide nanotubes on indium-doped tin oxide coated glass and silicon substrates The growth rate and diameter of the titanium dioxide nanotube arrays prepared in ethylene glycol containing electrolyte is larger than that in glycerol containing electrolyte The film thickness is calculated as 400 nm

In order to change the amorphous titanium dioxide into anatase titanium dioxide, the prepared titanium dioxide nanotube array film was annealed in air at 350ºC for an hour The annealed titanium dioxide electrode is used for preparing the dye-sensitized solar cell device The platinum-coated indium-doped tin oxide substrate is used as a counter electrode The photovoltaic parameters such as open circuit voltage (Voc), short-circuit current density (Jsc) and fill factor (FF) are calculated as 0.432 V, 1.58 mA/cm2 and 0.36, respectively The low value of fill factor is attributed to the large value of series resistance at the interface between titanium dioxide and indium-doped tin oxide films The efficiency of the prepared device is less than 1 % In this method, the film thickness is one of the disadvantages for DSC applications Because the amount of dye adsorption can be increased

as-by increasing the internal surface area as well as the thickness of the films

Fig 14 SEM images of Ti/ITO/glass and Ti/Si after anodization in glycerol containing 2.5 vol % H2O + 0.5wt % NH4F at 30 V and ethylene glycol containing 2.0 vol % H2O + 0.3wt

% NH4F at 20 V for 240 min (a) and 180 min (b), respectively

3.6 Preparation and characterization of zinc oxide nanorods on different substrates

There are many reports about fabrication and characterization of dye-sensitized solar cells However, the review results suggest that the recombination rate of the injected photoelectrons in dye-sensitized solar cell based on titanium dioxide electrode is very high compared to zinc oxide decorated titanium dioxide electrode, it is due to the absence of an energy barrier at the electrode to electrolyte interface In the present work, we study the effect of growth conditions on the surface morphological and structural properties of zinc oxide films We also investigate the photovoltaic performance of dye-sensitized solar cells based on titanium dioxide and titanium dioxide decorated with zinc oxide nanoparticles

Finally, discussion on possible factors that improve the dye-sensitized solar cell device performance, because two different kinds of photoelectrodes have been used in this study Nanostructured zinc oxide paste was prepared by using hydrothermal method In order to study the effect of substrates surface condition on the surface morphological properties of zinc oxide, zinc oxide films were also prepared on different substrates such as indium-doped tin oxide film coated flexible clay, glass, zinc plate and copper wire substrates Nanocrystalline indium-doped tin oxide films were prepared on clay and glass substrates

by ion beam sputter deposition method (Venkatachalam et al., 2011) The hydrothermal synthesis of zinc oxide paste and films were carried out in a Teflon-lined stainless steel autoclave In a typical synthesis process, zinc chloride (40 ml) was used with 2 ml of ammonia solution

by (Wang et al., 2008)

Figure 16A and B shows the scanning electron microscope images of zinc oxide films prepared on indium-doped tin oxide film coated glass and clay substrates The diameters of zinc oxide nanorods on both clay and glass substrates are not uniform; they are in the range

from hundred to several hundred nanometers The size of the zinc oxide nanorod on clay substrate is larger than that on glass substrate The growth parameters of zinc oxide films on both glass and clay were same The substrate surface roughnesses of indium-doped tin oxide film deposited on glass and clay were calculated by AFM The substrate surface roughnesses of ITO/glass and ITO/clay are calculated as 4.3 and 83 nm, respectively The substrate surface of ITO/clay is much larger than that of ITO/glass This is attributed that the substrate surface roughness strongly influences the growth rate of zinc oxide films X-ray diffraction pattern for zinc oxide film grown on glass shows a main peak at 2θ=34.76°, it corresponds to (002) orientation of hexagonal zinc oxide In contrast, the zinc oxide film deposited on clay shows a main peak at 2θ =32.08°, it corresponds to (100) plane X-ray diffraction patterns show two different orientations i.e., (002) and (100) on different substrates (glass and clay) (figure not shown) The exact reason, which determines the crystal growth and orientation, is the difference in substrate surface roughness between the glass and clay Figure 16C and D shows the scanning electron microscope images of zinc oxide nanorods synthesized by hydrothermal method on copper and zinc substrates The zinc oxide nanorods on both copper and zinc substrates are vertically oriented and well aligned (Fig 16C and D) It also reveals that the nanorods are grown in a very high density Scanning electron microscope images clearly show that the morphology of the final product

is strongly dependent on the substrate surface condition

Fig 16 Scanning electron microscope images of zinc oxide nanorods prepared on different substrates; (A) ITO/glass, (B) ITO/clay, (C) copper wire and (D) zinc plate

The titanium dioxide paste was coated on indium-doped tin oxide coated glass substrate by doctor blade method At first, the titanium dioxide coated ITO sample was annealed in air at

150°C for 30 min Then the annealed TiO2/ITO samples were placed into the zinc oxide solution for 30 sec Finally, all the samples were annealed in air at 400°C for 2 h The titanium dioxide film thicknesses are calculated as 1.5 and 3 μm In the present work, we employed a very thin layer of titanium dioxide film (1.5 or 3 μm) in order to check the effect of zinc oxide

on the performance of the dye-sensitized solar cells Finally, all the titanium dioxide electrodes were immersed into the ethanol solution containing ruthenium (N-719) dye Then the dye-anchored titanium dioxide electrodes were rinsed with ethanol solution and then dried in air

Figure 17 shows the photocurrent density-voltage characteristics of dye-sensitized solar cells based on titanium dioxide nanoparticulate film and zinc oxide decorated titanium dioxide films The short circuit density of titanium dioxide based dye-sensitized solar cell is lower than that of dye-sensitized solar cell based on zinc oxide decorated titanium dioxide (see Table 3) This is attributed that the titanium dioxide electrode introduces charge recombination that mainly occurs at the electrode/electrolyte, so that the open circuit voltage and fill factor values are low compared to zinc oxide decorated titanium dioxide, this is due to the absence of energy barrier layer (Wang et al., 2009) The performance of the dye-sensitized solar cell based

on zinc oxide decorated titanium dioxide has been improved; because the photogenerated electrons are more effectively extracted and, thereby, open circuit voltage (Voc), short-current density (Jsc) and fill factor (FF) increase together This is attributed that the protection of titanium dioxide surface with additional zinc oxide layer is considered to be another possible reason for the improved efficiency in zinc oxide decorated titanium dioxide photoanode This result indicates that the power conversion efficiency of dye-sensitized solar cell based on zinc oxide decorated titanium dioxide can be increased by increasing the titanium dioxide film thickness

-8.0x10-3-6.0x10-3-4.0x10-3-2.0x10-30.02.0x10-34.0x10-36.0x10-38.0x10-3

Fig 17 Photocurrent density-voltage characteristics of dye-sensitized solar cell based on

TiO2 and ZnO/TiO2 films

the (111) direction In the optical studies, the band gap value decreased from 2.72 to 2.60 eV as

the substrate temperature was increased from 483 to 589 K In the current–voltage studies, the

departure of the ideality factor from unity was due to the existence of a laterally varying

potential barrier height, caused by a non-uniform interface From the capacitance–voltage

study, the examined heterostructures are abrupt heterojunctions Indium-tin oxide thin films were deposited on clay and glass substrates by ion beam sputter deposition method at room temperature The flexibility of indium doped tin oxide coated clay substrate was measured as

17 mm The as-deposited indium doped tin oxide coated films on flexible clay substrate showed low sheet resistance (41 Ω/…) and high optical transmittance (∼80%) Titanium dioxide nanorods were prepared on indium doped tin oxide coated glass and fluorine doped tin oxide coated glass substrates by hydrothermal method The titanium dioxide nanorods were grown perpendicular to the fluorine doped tin oxide substrate; it was attributed to epitaxial growth of titanium dioxide films Finally, flexible dye-sensitized solar cell was successfully fabricated The titanium dioxide nanotube arrays and nanowires covered titanium dioxide nanotube arrays were successfully prepared by electrochemical anodization method

In this case, the dye adsorption capacity and power conversion efficiency of dye-sensitized solar cells based on nanowire covered titanium dioxide nanotube arrays were much higher than that of dye-sensitized solar cells based on titanium dioxide nanotube arrays The titanium films were deposited on indium doped tin oxide coated glass substrate The titanium dioxide nanotube arrays were successfully prepared on titanium films at room temperature Nanostructured zinc oxide films were successfully deposited on different substrates by hydrothermal method X-ray diffraction study clearly showed that the crystal quality and orientation of the final products were strongly dependent on the experimental parameter Scanning electron microscope images showed that the shape and size of the nanorods could be perfectly generated by controlling the substrate surface roughness The efficiency of ZnO/TiO2

based DSC significantly improved from 0.9 to 2 % as the titanium dioxide film thickness was increased from 1.5 to 3μm It showed the positive role of zinc oxide coating that leads to the improvement of the efficiency This result indicated that the zinc oxide coating on the titanium dioxide surface suppresses the recombination at the TiO2/dye/electrolyte interface The power conversion efficiency could be increased by increasing the TiO2 film thickness

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